7.1 Describing Reactions
Burning is a chemical
change. When a
substance undergoes a
chemical change, a
chemical reaction is
said to take place.
7.1 Describing Reactions
Chemical Equations
What is the law of conservation of mass?
The law of conservation of mass states that
mass is neither created nor destroyed in a
chemical reaction.
7.1 Describing Reactions
Chemical Equations
A useful description of a chemical reaction tells
you the substances present before and after
the reaction.
• The substances that undergo change are called
reactants.
• The new substances formed as a result of that
change are called products.
7.1 Describing Reactions
Chemical Equations
Using Equations to Represent Reactions
During a chemical reaction, the reactants change
into products. You can summarize this process
with a word equation.
Reactants  Products
7.1 Describing Reactions
Chemical Equations
To describe the burning of charcoal, you can
write reactants and products of the reaction in
the following word equation.
Carbon + Oxygen  Carbon dioxide
Write the reactants and products as chemical
formulas.
C + O2  CO2
7.1 Describing Reactions
Chemical Equations
A chemical equation is a representation of a
chemical reaction in which the reactants and
products are expressed as formulas. You can
read the equation C + O2  CO2 as
• “ Carbon and oxygen react and form carbon
dioxide,” or,
• “The reaction of carbon and oxygen yields
carbon dioxide.”
7.1 Describing Reactions
Chemical Equations
Conservation of Mass
During chemical reactions, the mass of the
products is always equal to the mass of the
reactants. This principle is the law of conservation
of mass.
When charcoal burns, the mass of the carbon
dioxide produced is equal to the mass of the
charcoal and oxygen that reacted.
7.1 Describing Reactions
Chemical Equations
Whether you burn one carbon atom or six
carbon atoms, the equation used to describe
the reaction is always the same.
7.1 Describing Reactions
Chemical Equations
Whether you burn one carbon atom or six
carbon atoms, the equation used to describe
the reaction is always the same.
7.1 Describing Reactions
Chemical Equations
Whether you burn one carbon atom or six
carbon atoms, the equation used to describe
the reaction is always the same.
7.1 Describing Reactions
Balancing Equations
Why must chemical equations be balanced?
In order to show that mass is conserved
during a reaction, a chemical equation must
be balanced.
7.1 Describing Reactions
Balancing Equations
You can balance a chemical equation by
changing the coefficients, the numbers that
appear before the formulas.
When you change a coefficient, you change
the amount of that reactant or product
represented in the chemical equation.
7.1 Describing Reactions
Balancing Equations
Water is a compound made up of the elements
hydrogen and oxygen.
7.1 Describing Reactions
Balancing Equations
You can balance a chemical equation by
changing the coefficients, the numbers that
appear before the formulas.
When you change a coefficient, you change
the amount of that reactant or product
represented in the chemical equation.
7.1 Describing Reactions
Balancing Equations
7.1 Describing Reactions
Balancing Equations
7.1 Describing Reactions
Balancing Equations
The chemical equation for the formation of
water is now balanced.
• Each side of the balanced equation has four
hydrogen atoms and two oxygen atoms.
• According to the balanced equation, two
molecules of hydrogen react with one molecule
of oxygen to yield two molecules of water.
7.1 Describing Reactions
Balancing Equations
Balancing Chemical Equations
Write a balanced equation for the reaction
between copper and oxygen to produce copper(II)
oxide, CuO.
7.1 Describing Reactions
Balancing Equations
Read and Understand
What information are you given?
7.1 Describing Reactions
Balancing Equations
Read and Understand
What information are you given?
Reactants: Cu, O2 Product: CuO
7.1 Describing Reactions
Balancing Equations
Plan and Solve
Write a chemical equation with the reactants
on the left side and the product on the right.
7.1 Describing Reactions
Balancing Equations
Plan and Solve
Write a chemical equation with the reactants
on the left side and the product on the right.
Cu + O2  CuO
7.1 Describing Reactions
Balancing Equations
Cu + O2  CuO
This equation is not balanced. Change the coefficient
of CuO in order to balance the number of oxygen
atoms.
7.1 Describing Reactions
Balancing Equations
Cu + O2  CuO
This equation is not balanced. Change the coefficient
of CuO in order to balance the number of oxygen
atoms.
Cu + O2  2CuO
7.1 Describing Reactions
Balancing Equations
Cu + O2  CuO
This equation is not balanced. Change the coefficient
of CuO in order to balance the number of oxygen
atoms.
Cu + O2  2CuO
Change the coefficient of Cu in order to balance the
number of copper atoms.
7.1 Describing Reactions
Balancing Equations
Cu + O2  CuO
This equation is not balanced. Change the coefficient
of CuO in order to balance the number of oxygen
atoms.
Cu + O2  2CuO
Change the coefficient of Cu in order to balance the
number of copper atoms.
2Cu + O2  2CuO
7.1 Describing Reactions
Balancing Equations
Look Back and Check
Is your answer reasonable?
7.1 Describing Reactions
Balancing Equations
Look Back and Check
Is your answer reasonable?
The number of atoms on the left equals the number
of atoms on the right.
7.1 Describing Reactions
Describing Ionic Compounds
1. Hydrogen chloride, or HCl, is an important
industrial chemical. Write a balanced equation for
the production of hydrogen chloride from
hydrogen and chlorine.
7.1 Describing Reactions
Describing Ionic Compounds
1. Hydrogen chloride, or HCl, is an important
industrial chemical. Write a balanced equation for
the production of hydrogen chloride from
hydrogen and chlorine.
Answer: H2 + Cl2  2HCl
7.1 Describing Reactions
Describing Ionic Compounds
2. Balance the following chemical equations.
a. H2O2  H2O + O2
b. Mg + HCl  H2 + MgCl2
7.1 Describing Reactions
Describing Ionic Compounds
2. Balance the following chemical equations.
a. H2O2  H2O + O2
b. Mg + HCl  H2 + MgCl2
Answer:
a. 2H2O2  2H2O + O2
b. Mg + 2HCl  H2 + MgCl2
7.1 Describing Reactions
Describing Ionic Compounds
3. Ethylene, C2H4, burns in the presence of
oxygen to produce carbon dioxide and water
vapor. Write a balanced equation for this reaction.
7.1 Describing Reactions
Describing Ionic Compounds
3. Ethylene, C2H4, burns in the presence of
oxygen to produce carbon dioxide and water
vapor. Write a balanced equation for this reaction.
Answer: C2H4 + 3O2  2CO2 + 2 H2O
7.1 Describing Reactions
Counting With Moles
Why do chemists use the mole?
Because chemical reactions often involve
large numbers of small particles, chemists
use a counting unit called the mole to
measure amounts of a substance.
A mole (mol) is an amount of a substance that
contains approximately 6.02 × 1023 particles.
For instance, a mole of iron is 6.02 × 1023 atoms
of iron.
This number is known as Avogadro’s number.
7.1 Describing Reactions
Counting With Moles
You might count shoes by the pair, eggs by the
dozen, or paper by the ream (500 sheets). To
count particles of a substance, chemists use the
mole (6.02 × 1023 particles).
7.1 Describing Reactions
Counting With Moles
Molar Mass
The mass of one mole of a
substance is called a molar
mass. For an element, the
molar mass is the same as its
atomic mass expressed in
grams.
• The molar mass of carbon is
12.0 grams.
• The molar mass of sulfur is
32.1 grams.
7.1 Describing Reactions
Counting With Moles
You calculate the molar mass of a compound by
adding up the atomic masses of its component
atoms and then expressing this sum in grams.
• A carbon dioxide molecule is composed of one
carbon atom (12.0 amu) and two oxygen atoms (2 ×
16.0 amu = 32.0 amu).
• Carbon dioxide has a molar mass of 44.0 grams.
7.1 Describing Reactions
Counting With Moles
Mole-Mass Conversions
Once you know the molar mass of a substance,
you can convert moles of that substance into
mass, or a mass of that substance into moles.
The molar mass of CO2 is 44.0 grams, which
means that one mole of CO2 has a mass of 44.0
grams. This relationship yields the following
conversion factors.
7.1 Describing Reactions
Counting With Moles
Suppose you have 55.0 grams of CO2. To
calculate how many moles of CO2 you have,
multiply the mass by a conversion factor.
You can check your answer.
7.1 Describing Reactions
Chemical Calculations
How can you calculate the mass of a
reactant or product in a chemical reaction?
In chemical reactions, the mass of a reactant
or product can be calculated by using a
balanced chemical equation and molar
masses of the reactants and products.
7.1 Describing Reactions
Chemical Calculations
A cake recipe tells you how much of each
ingredient to use for each cake. Chemical
equations can be read as recipes for making new
substances.
7.1 Describing Reactions
Chemical Calculations
In a balanced chemical equation, the number of
atoms of each element on the left equals the
number of atoms of each element on the right.
7.1 Describing Reactions
Chemical Calculations
In a balanced chemical equation, the number of
atoms of each element on the left equals the
number of atoms of each element on the right.
7.1 Describing Reactions
Chemical Calculations
In a balanced chemical equation, the number of
atoms of each element on the left equals the
number of atoms of each element on the right.
7.1 Describing Reactions
Chemical Calculations
In a balanced chemical equation, the number of
atoms of each element on the left equals the
number of atoms of each element on the right.
7.1 Describing Reactions
Chemical Calculations
Converting Mass to Moles
To calculate how much oxygen is required to
make 144 grams of water, begin with a balanced
chemical equation for the reaction.
2H2 + O2  2H2O
• Determine how many moles of water you are trying to
make.
• Convert the given mass of water into moles.
7.1 Describing Reactions
Chemical Calculations
Using Mole Ratios
• Write the conversion factors, or mole ratios.
• Calculate how many moles of oxygen are
required to produce eight moles of water.
7.1 Describing Reactions
Chemical Calculations
Converting Moles to Mass
• Convert moles of O2 to grams of O2 by using
the molar mass of O2 as a conversion factor.
• To produce 144 grams of H2O, you must supply
128 grams of O2.
7.1 Describing Reactions
Assessment Questions
1. Which of the following is a balanced chemical
equation for the reaction of aluminium and
ammonium perchlorate?
a.
b.
c.
d.
Al + NH4ClO4 —› Al2O3 + NH4Cl
4Al + 3NH4ClO4 —› 4Al2O3 + 3NH4Cl
8Al + NH4ClO4 —› 4Al2O3 + NH4Cl
8Al + 3NH4ClO4 —› 4Al2O3 + 3NH4Cl
7.1 Describing Reactions
Assessment Questions
1. Which of the following is a balanced chemical
equation for the reaction of aluminium and
ammonium perchlorate?
a.
b.
c.
d.
Al + NH4ClO4 —› Al2O3 + NH4Cl
4Al + 3NH4ClO4 —› 4Al2O3 + 3NH4Cl
8Al + NH4ClO4 —› 4Al2O3 + NH4Cl
8Al + 3NH4ClO4 —› 4Al2O3 + 3NH4Cl
ANS: D
7.1 Describing Reactions
Assessment Questions
2. How many moles of NaCl (molar mass = 58.5) are
there in a 10-gram sample?
a.
b.
c.
d.
58 mol
5.8 mol
0.17 mol
0.28 mol
7.1 Describing Reactions
Assessment Questions
2. How many moles of NaCl (molar mass = 58.5) are
there in a 10-gram sample?
a.
b.
c.
d.
58 mol
5.8 mol
0.17 mol
0.28 mol
ANS: C
7.1 Describing Reactions
Assessment Questions
3. In the reaction shown below, how many grams of
hydrogen are produced when 10 moles of
potassium react with water?
2K + 2H2O —› 2KOH + H2
a.
b.
c.
d.
2.5 g
5g
10 g
20 g
7.1 Describing Reactions
Assessment Questions
3. In the reaction shown below, how many grams of
hydrogen are produced when 10 moles of
potassium react with water?
2K + 2H2O —› 2KOH + H2
a.
b.
c.
d.
2.5 g
5g
10 g
20 g
ANS: C
7.1 Describing Reactions
Assessment Questions
1. In the chemical equation below, the reactants are
carbon dioxide and water.
CH4 + 2O2 —› CO2 +2H2O
True
False
7.1 Describing Reactions
Assessment Questions
1. In the chemical equation below, the reactants are
carbon dioxide and water.
CH4 + 2O2 —› CO2 +2H2O
True
False
ANS:
F, products